ARTICLE IN PRESS

WAT E R R E S E A R C H 4 1 ( 2 0 0 7 ) 1 8 9 7 – 1 9 0 61898

et al., 2006). Consequently, research in the development of

effective and low-cost heavy metals and radionuclide sor-

bents is not only scientifically and economically attractive but

also urgent for environmental and health protection.

Naturally occurring clay minerals have been extensively

studied as sorbents for the removal of various pollutants from

wastewater and aqueous solutions (Al-Qunaibit et al., 2005).

Montmorillonite, coated and intercalated by aluminum

hydroxides, exhibits much higher adsorption capacity for

some heavy metal ions than that of natural montmorillonite

(Lothenbach et al., 1997). High-temperature calcination of

intercalated clays results in ‘‘pillared’’ materials, where the

polyhydroxo-cationic species are irreversibly fixed to the

layers. Pillared montmorillonites, first tested as cracking

catalyst in view of their acidic properties, have recently been

studied as sorbents for the removal of hazardous inorganic

elements (Karamanis et al., 1997; Cooper et al., 2002; Jiang and

Zeng, 2003; Manohar et al., 2005, 2006). However, very little is

still known about the removal efficiency and the sorption

mechanisms of metals by pillared clays (Manohar et al., 2005,

2006). Therefore, the present work investigated the efficiency

of aluminum-pillared-layered montmorillonites (PILMs) for

the removal of copper and cesium from aqueous solutions

under kinetic and equilibrium conditions. In order to identify

the mechanisms of copper and cesium sorption on pillared

montmorillonites, the pillaring conditions were varied and

different forms of PILMs were prepared and tested in sorption

experiments.

2. Materials and methods

2.1. Preparation of PILMs

Bentonite was provided by Silver & Baryte Min. Co., Athens.

The montmorillonite fraction of the starting bentonite was

brought to its Na-homoionic form (Na-montmorillonite,

NaM). Specifically, bentonite was repeatedly contacted with

1 M NaCl solutions and washed with deionized water until Cl�

free (AgNO3 test). Then, a 1 wt% aqueous suspension of the

treated bentonite was prepared and left to stand for 24 h. The

upper 80% of the colloid column was then separated,

centrifuge-washed and finally gently ground after drying at

room temperature.

An aluminum-pillared montmorillonite, coded Al1MFD3,

was prepared according to the method described by Karama-

nis et al. (1997). Briefly, the Naþ form of montmorillonite was

added to the pillaring solution (a 0.2 M solution of AlCl3 with a

0.2 M solution of NaOH up to a molar ratio OH/Al of 1) at a

ratio of 3 mmol Al per gram clay and left to react for 1 h under

vigorous stirring at 70 �C. Once the exchange process was

completed, the PILM precursor was repeatedly washed and

then freeze-dried. Heating the PILM precursor in air at 500 �C

for 4 h in order to create the rigid, non-swelling, three-

dimensional zeolite-like structure (Varma, 2002), delivered

the final PILM in powder form. Its cation-exchange capacity

was restored by contacting the powder with an ammonia

atmosphere and then with a NaCl solution at pH 10

(Karamanis et al., 1997). The PILM material was recovered by

centrifugation and was subsequently centrifuge-washed

1

several times with deionized water; it was finally dried at

80 �C. PILMs coded as AlXMFDY were prepared accordingly by

varying the OH/Al ratio (X ¼ 0:125, 1 or 2) and the ratio of

added mmol Al per gram of clay (Y ¼ 3, 5, 15).

2.2. Materials characterization

2.2.1. Elemental analysisThe concentration of each element in the prepared materials

was determined with the spectrometric methods of proton

induced gamma-ray emission (PIGE) and X-ray fluorescence

(XRF) (Karamanis et al., 2001). PIGE measurements were

carried out at the 5.5 mV terminal voltage TN11 Tandem

accelerator of the National Center for Scientific Research

‘‘Demokritos’’ in Athens. A proton beam with energy of Ep ¼

4:0 MeV was used and the emitted g-radiation was detected

with an 80% high purity germanium (HPGe) detector. Char-

acteristic g-rays emitted from the deexcitation of the residual

nuclei following (p,p0g) reactions, were used for the determi-

nation of light elements as Al (1014 keV), Si (1779 keV), Mg

(585 keV) and Na (440 keV). For the normalization and extrac-

tion of weight percentages, samples of the certified reference

material IAEA Soil-7 and of powder graphite mixed with

cellulose and a compound containing the element under

investigation (in its natural isotopic abundance), were used as

standards.

XRF measurements were performed at the XRF Unit of the

University of Ioannina. A vertical Si(Li) detector was used and

the exciting radiation was provided by a ring shaped radio-

isotope source (109Cd or 241Am). Samples in the form of pellets

were placed at the top of the assembly in a pi geometry

between the exciting radiation and the samples. Induced

X-rays were detected through a small hole in the shielding

material. Normalization was performed with direct compar-

ison with the reference IAEA material Soil-7 and matrix

effects were corrected through simulation.

2.2.2. X-ray diffraction (XRD)XRD patterns of PILM materials were collected on a Bruker

AXS D8 Advance Bragg–Brentano geometry with Cu sealed-

tube radiation source (l ¼ 1:54178 A) plus a secondary beam

graphite monochromator. A step of 0:02� and a time of 3 s

step�1 were selected.

2.3. Sorption experiments

2.3.1. Cesium sorption studiesThe uptake of Csþ was investigated via equilibration and

exchange kinetic measurements. The conventional batch

technique was employed for the equilibration measurements

in which a known amount of material was contacted with a

solution containing chloride salts of Csþ, traced with 137Cs.

After attaining equilibrium, the two phases were separated by

centrifugation and the g activity of the supernatant was

measured with a 22% HPGe detector. In addition to batch

kinetic measurements, a dynamic dialysis method was also

used (Karamanis et al., 1997). Periodically removing the

dialysis bag and measuring the remaining solution activity

determined the rate of cation uptake.

ARTICLE IN PRESS

WAT E R R E S E A R C H 41 (2007) 1897– 1906 1899

2.3.2. Copper sorption studiesThe aqueous solutions of Cu2þ were obtained from dilutions

of stock solutions prepared from dissolving CuCl2. All

sorption experiments were conducted in mechanically stirred

glass reactors equipped with a thermometer and a pH

electrode to measure variations in temperature and pH. At

equilibrium, PILMs were filtered through a 0:47mm millipore

filter. Sorption kinetics were studied with the batch or the

dialysis method. In each experiment, duplicate samples of

1 mL from the sorption solution were analyzed by XRF prior to

materials immersion in the solution and till equilibrium

attainment. The volume of 1 mL of solution was pipetted onto

a 12 mm Whatman No. 42 filter (held in a mylar film of 6:3mm

thick) under continuous drying with an IR-lamp and was

directly XRF measured. The quantity of metal ion sorbed on

each material was determined by the difference between the

initial metal concentration and the remaining concentration

at equilibrium. XRF was calibrated with standards, which

were prepared by diluting known volumes of the standard

stock solutions in deionized water. Metal concentrations of

1.0–100:0 mg L�1 were used and four replicates were prepared

of each. The calibration curve was linear in all the studied

region and with a minimum detection limit of 0:09mg of

copper on filter.

Fig. 1 – XRD patterns of sodium montmorillonite (NaM) and

the prepared pillared samples (01MFD3 is the sample with

OH/Al ratio of 0.125 and Al/clay ratio of 3 mmol g�1, 1MFD3

3. Results and discussion

3.1. PILMs characterization

Elemental analysis of NaM and PILMs is shown in Table 1. The

increase in aluminum weight percentage between NaM and

the PILM derivatives was dependent on the initial aluminum

availability per gram of clay. It is known that the Al/clay ratio

has a significant effect on the accessibility properties in Al-

pillared clays. With an Al/clay ratio of higher than 5 mmol g�1,

a bimodal micropore distribution has been observed

with peak dimensions at 0.4 and 0.55 nm (Hutson et al.,

1999). Moreover, higher initial aluminum availability leads

to a higher pillar density (Gil and Montes, 1995) and

therefore higher fraction of pores with an opening of less

1

Table 1 – Elemental analysis of Na-montmorillonite (NaM) and

(%) NaM Al1aMFD3b A

SiO2c 55.9 52.2

Al2O3c 18.6 24.3

Fe2O3d 2.9 2.9

MgOd 2.9 2.7

CaOc 0.1 0.1

K2Od 0.5 1.0

Na2Oc 2.5 2.2

a Molar OH/Al ratio.b Al/clay ratio (mmol g�1Þ.c PIGE.d XRF.

than 0.45 nm. In this way, the effect of the pillar density or the

interpillar distance on cesium or copper sorption was further

studied.

The grain size of all the prepared PILMs was measured to be

lower than 45mm. The variation of the OH/Al ratio resulted in

different d001-spacing of the pillared samples as deduced from

the PILMs XRD patterns (Fig. 1). The d001 peak was narrower

and more intense in Al2MFD15 (1.75 nm) and Al2MFD5

(1.74 nm) than in the other two pillared samples Al2MFD3

(1.72 nm) and Al1MFD3 (1.67), indicating a more ordered or

higher crystallinity in these samples. The OH/Al ratio of 0.125

did not result in a pillared sample and thus, the sample was

not used in the subsequent sorption tests. All other samples

were tested in sorption experiments and the influence of

PILMs interlayer spacing in their sorption ability was further

investigated.

its aluminum pillared products

l2aMFD3b Al2aMFD5b Al2aMFD15b

52.7 52.3 52.4

24.5 25.3 28.5

2.9 2.9 2.9

2.6 2.6 2.4

0.1 0.1 0.1

1.3 1.1 1.2

2.2 2.2 2.4

is Al1MFD3 with an OH/Al ratio of 1 and Al/clay ratio of

3 mmol g�1, etc.).

ARTICLE IN PRESS

Fig. 3 – Effect of pH on cesium sorption (1:7� 10�11 M) on

Al2MFD15 (0:15 g L�1).

WAT E R R E S E A R C H 4 1 ( 2 0 0 7 ) 1 8 9 7 – 1 9 0 61900

3.2. Cesium sorption experiments

3.2.1. Cesium sorption on different PILMsEven in the case of a nuclear accident, the concentration of

cesium ions in the surface waters is extremely low (less than

1 ppb). For example, 1 MBq L�1 137Cs is equivalent to

6:8� 10�9 M. Therefore, the prepared PILMs were tested for

their ability to sorb cesium in a solution of natural mineral

water (0.2 L) with cationic concentrations of Ca: 1� 10�3 M,

Mg: 0:65� 10�4 M, Na: 0:75� 10�4 M, K: 0:2� 10�4 M and a

small added radiocesium concentration (1:7� 10�11 M). In

these experiments, the cesium removal for a contact time of

1.5 h was measured to be around 75� 2% for all the four

pillared samples (20 mg). Therefore, either the d001 interlayer

spacing or the interpillar distance had no effect in cesium

sorption from dilute concentrations.

3.2.2. Cesium sorption kineticsDifferent kinetic models such as the Lagergen’s pseudo-first-

order equation, second-order equation and Elovich equation

were tested to find out which model is in agreement with the

experimental results of the kinetic study. On comparison, the

pseudo-first-order rate equation yielded the best results for

cesium sorption on PILMs. It can be represented as

dqdt¼ k1 � ðqe � qÞ. (1)

Integrating Eq. (1) for the boundary condition qt ¼ 0 at t ¼ 0,

the equation becomes

qt ¼ qe � ð1� e�k1�tÞ. (2)

The rate constant k1 (min�1Þ and the equilibrium capacity,

qe ðmmol g�1Þ were determined as free parameters by a ‘‘best-

fit’’ minimization of the corresponding w2 function. The

kinetic isotherms and their corresponding fitting results in

Fig. 2 showed that Csþ sorption in PILMs is a fast process and

is accomplished in less than 30 min. The rate constant was

the same for the different PILMs while the equilibrium

capacity was higher for the Al2MFD15 material due to a

higher restoration of the cation-exchange capacity of the

initial NaM material. A similar behavior was also observed in

kinetics conducted in the presence of the competing cations

of Kþ or Ca2þ.

1

Fig. 2 – Cesium (1 mM) sorption kinetics by Al1MFD3 and

Al2MFD15 (0:5 g L�1).

3.2.3. Effect of pHThe influence of pH on the sorption of cesium was examined

in the pH range from 3.0 to 8.0. As shown in Fig. 3, the

percentage of sorbed cesium increased with increasing pH

with a plateau region from around 4.5 to 8.0. The decrease in

the Cs uptake observed at low pH values can be partly

attributed to the positive charge, which develops on the PILM

edges according to the process noted by Avena et al. (1990)

SO� þHþ ! SOH and SOHþHþ ! SOHþ2 where S stands for

any surface site.

However, it has previously been reported (Delgado et al.,

1986) that the clay sheets of montmorillonites remain

negatively charged at pH values down to 3, since the positive

charge of the edges does not exceed the charge of the

sheets. It can therefore be concluded that the Cs uptake at

acidic pH values is suppressed, because the remaining

negative charge of the PILM is preferentially compensated

by H3Oþ ions.

Hydronium ions, although monovalent, behave mostly

as di or trivalent ion. The same behavior is also usually

observed in non-pillared exchanging clays (Grim, 1968). In

contrast, the observed increase of cesium sorption with

increasing pH can be attributed to the decrease of the

competition of the hydronium ions for PILMs’ sites at higher

pH values.

3.2.4. Cesium sorption isothermsEquilibrium sorption studies were performed to determine

the maximum cesium sorption capacity of PILMs in the

availability range of 0.4–10 mmol cesium per gram of PILMs in

200 mL aqueous solution.

Cesium uptake was quantitatively evaluated using the one

or two-site Langmuir model:

qe ¼Xmi¼1

Ki � Ce

1þ Ki � CeQi, (3)

where qe is the adsorption capacity at equilibrium (mg g�1Þ, Ce

is the equilibrium concentration of metal ions in the solution

(mg L�1Þ, m is the number of energetically different sorption

sites, Qi and Ki are the adsorption capacities and binding

strengths of the adsorbed cations, respectively.

ARTICLE IN PRESS

Table 2 – Freundlich and Langmuir isotherm values andparameters for the sorption of Csþ onto Al2MFD15sample

Values and parameters Magnitude

Freundlich

KF (L g�1Þ 1:40� 0:08

1=n 0:23� 0:04

w2 40.53

One– site Langmuir

Q ðmg g�1Þ 82:64� 2:05

K ðL mg�1Þ 0:10� 0:01

w2 5.381

SD(%) 5.68

Two-site Langmuir

Q1 ðmg g�1Þ 18:56� 26:30

K1 ðL mg�1Þ 0:74� 0:85

Q2 ðmg g�1Þ 66:12� 25:32

K2 ðL P mg�1Þ 0:066� 0:034

w2 4.95

SD(%) 2.52

Fig. 4 – Comparison of the Langmuir and Freundlich

isotherms for the sorption of cesium onto NaM and

Al2MFD15 (qe is the adsorption capacity at equilibrium

(mg g�1) and Ce is the equilibrium concentration of metal in

the solution (mg L�1)).

WAT E R R E S E A R C H 41 (2007) 1897– 1906 1901

The Freundlich model was also used as

log qe ¼ log KF þ1n

log Ce, (4)

where qe and Ce as described in the Langmuir model, b and n

are constants related to the energy of adsorption and KF is a

constant related to sorption capacity.

The values of normalized standard deviation (SD(%)) were

calculated using the equation

SDð%Þ ¼ 100�

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiP½ðqexp

e � qcale Þ=q

expe �

2

N� 1

s, (5)

where the superscripts ‘‘exp’’ and ‘‘cal’’ are the experimental

and calculated values with the fitted parameters and N is the

number of measurements.

The values of the isotherm constants in Table 2 were

determined by fitting the above equations to the experimental

data. To assess the different isotherms and their validity to

correlate experimental results, the experimental data and the

theoretical plots for each isotherm are shown in Fig. 4. The

one-site Langmuir represented better the cesium sorption

data than the Freundlich isotherm (Table 2), suggesting the

monolayer sorption, mainly due to ion exchange. The w2 and

(SD(%)) values for the one-site Langmuir model were very low,

indicating a very good mathematical fit. The cesium sorption

capacity of PILMs was determined to be less than that of the

starting clay and this can be attributed to the remaining

charge of aluminum pillars and/or the incomplete cation-

exchange capacity restoration. As it was observed in kinetic

isotherms, the maximum cesium sorption capacity of the

PILM materials was obtained with sample Al2MFD15.

Moreover, the two-site Langmuir model fitted the experi-

mental results even better, indicating the existence of

heterogeneity. The two-site Langmuir constants for the

sorption of Csþ showed that high-affinity sites exist with a

1

low sorption maximum (20%) and low-affinity sites exist with

a high sorption maximum (80%). However, the error in the

estimated parameters was high and the exact amount of

different sites could not be safely concluded. Therefore, the

surface heterogeneity of PILMs was further investigated by

varying the initial cesium concentration in the presence of

competitive cations.

3.2.5. PILMs selectivity for cesium sorptionCesium selectivity of the PILM materials was studied for the

binary systems of Csþ=Naþ, Csþ=Kþ and Csþ=Ca2þin several

ionic concentrations from dilute up to highly concentrated

solutions of cesium and competitive cations. In all systems,

materials were initially transformed in the form of the

competing cation.

Assuming the general exchange reaction

zst;AAþ zst;bB 2 zst;AAþ zst;BB (6)

the cesium selectivity was studied through:

A.

Plots of the equivalent fractions of cesium sorbed in PILMs

(xCsÞ vs. their equivalent fraction in the solution (xCsÞ at

equilibrium for constant solution ionic strength (INÞ.

B.

Variation of the distribution coefficient Kd defined as the

ratio of concentrations of Cs sorbed xc, (mol kg�1Þ and in

the solution [Cs] (mol L�1Þ.

C.

Variation of the selectivity coefficient Kc defined as

Kc ¼ðYAÞ

zst;A

ðYBÞzst;B�ðMBÞ

zst;B

ðMAÞzst;A�ðgBÞ

zst;B

ðgAÞzst;A

, (7)

where YA are the equivalent fraction of ions in the solid,

MA the molality of ions in the solution and gA the activity

coefficients in the liquid phase (Ioannidis et al., 2000).

These were calculated with the extended Debye-Huckel

equation.

Starting from the first representation, the results in Fig. 5

clearly demonstrate that PILMs exhibit a high selectivity for

Csþ sorption over Kþ and much higher over Naþ. By varying

the initial concentration of Cs in the solution (10�8210�4 MÞ, a

ARTICLE IN PRESS

WAT E R R E S E A R C H 4 1 ( 2 0 0 7 ) 1 8 9 7 – 1 9 0 61902

decrease of the distribution coefficient Kd was observed and

shown in Fig. 6. The variation of Kd with the initial Cs

concentration shows that the adsorption isotherm is not

linear (Staunton and Roubaud, 1997). The decrease of the Kd

coefficient was much higher in the system Cs–Na-PILM than

the Cs–K-PILM and this order was observed at all levels of Csþ,

Naþ or Kþ concentrations. In the same figure, the variation of

the distribution coefficient for two constant cesium concen-

trations as a function of the concentration of the competitive

Naþ or Kþ (2� 10�420:5� 10�1 M) is also included. The slope

of the Kd reduction was greater for K-PILM than Na-PILM for

the two constant cesium concentrations. However, the Kd

parameter is not a sensitive measure of the relative sensitiv-

ities of a material for various cations since the activity

coefficients and the stoichiometry of the sorption process are

not included in the Kd definition (Staunton and Roubaud,

1997). To overcome this weakness, the variation of the

selectivity coefficient Kc as a function of the sorbed cesium

fraction (zCs) in PILMs was studied. As shown in Fig. 7 for the

Fig. 5 – Cs–Na and Cs–K selectivity isotherms on PILMs in

aqueous solutions of total ionic strength of 10�3.

Fig. 6 – Variation of the distribution coefficient Kd as a fun

1

sodium form of pillared clays, values of the selectivity

coefficient were high for 1–2% of site occupancy and started

to decrease with increasing Cs concentration and higher

sorption. Similar results were also observed with potassium

as the interlayer exchangeable cation. These results indicate

that energetically less favorable sites become involved with

increasing cesium concentration in the aqueous solution.

Such behavior is typical for heterogeneous materials with

sites of different selectivity (Zachara et al., 2002). In PILMs, the

selectivity values for this small fraction of sites are much

higher than those of the starting NaM material and compar-

able to those found in materials with high cesium selectivity

as ferrocyanides (Lin et al., 2001), a small fraction of the total

sites of illite (Staunton and Roubaud, 1997; Zachara et al.,

2002) and titanosilicates (Solbra et al., 2001). The fraction of

the highly selective sites of PILMs for cesium sorption can be

attributed either to micropores with a suitable opening for

cesium uptake (around 0.4 nm according to Hutson et al.,

1998) or to collapse of the aluminosilicate sheets and creation

of sites similar to illite.

ction of cesium, sodium or potassium concentration.

Fig. 7 – Variation of the selectivity coefficient Kc as a function

of the cesium fraction in PILMs for sodium solution of

1� 10�3 M.

ARTICLE IN PRESS

WAT E R R E S E A R C H 41 (2007) 1897– 1906 1903

In order to clarify the origin of PILMs heterogeneity,

different blocking agents were used to isolate the PILMs’

group sites. Lithium ions were used to block any sites

associated with the clay’s hexagonal cavities. Silver thiourea

ions (AgTUþÞ, formed from a mixture of SCðNH2Þ2 and AgNO3

in a molar ratio of 3:1, were used to mask the regular

interlamellar (planar) sites (Cremers et al., 1988). Moreover, it

has been observed that the crystal violet (CV) cation is

strongly attached at the surface of PILMs since its diffusion

is hindered in PILMs interlammelar space due to steric

reasons (Mishael et al., 1999). Therefore, CV was used to

block the surface and edge sites. Prior to the experiments

with blocking agents all pillared samples were saturated and

transformed in the form of the blocking agent in concentra-

tions of at least 10-fold of PILMs cesium sorption capacity.

Cesium sorption isotherms were measured by equilibrating

the agent-saturated PILMs with cesium solutions of varying

initial concentration in a background ion concentration of

1� 10�3 M.

As seen in Fig. 8, the values of the selectivity coefficient

Cs–Na for the NaM montmorillonite are in agreement with

those usually observed (Staunton and Roubaud, 1997). The

values of the selectivity coefficient Cs–Na for the PILMs

material are much higher than those of the starting clay in

all the cesium concentration range. The values of the

selectivity coefficient Cs–Li follow the same behavior as those

of the Cs–Na selectivity and are almost the same. Therefore,

the high-affinity sites are not related with the hexagonal

Fig. 8 – Variation of the selectivity coefficient Kc as a function

of the cesium fraction in Al1MFD3 for different blocking

agents of 1� 10�3 M.

Table 3 – Copper sorption from aqueous solutions by different

NaM

Cu sorbed (%) 60:5� 2:3

Cu sorbed (meq g�1)/Cs sorption capacityb (meq g�1Þ 0.75

The solution/solid ratio ðV=MÞ ¼ 1 L g�1. pH ¼ 4:8.a Al2MFD15-Un is the sample that the cation-exchange capacity was nob Cs sorption capacity is assumed to be equal to the cation-exchange ca

1

cavities of the clay structure. The values of the selectivity

coefficient Cs–Ca for 1–2% of site occupancy are comparable

to the very high values observed in a 0.1% fraction of illite

sites. The silver thiourea cation does not affect the high

affinity sites for cesium sorption but reduces the low affinity

sites in the same manner as in illite (Zachara et al., 2002). The

CV cation reduces all sites by strongly limiting the high-

affinity sites but also blocking (due to the formation of

molecular aggregates on the outer surface of the clay (Mishael

et al., 1999)) the pores entrance and thus reducing the low-

affinity sites as well.

Concluding, the results of the present study provide direct

evidence that the high selectivity of PILMs for cesium sorption

is rather related to their surface and edge sites than to

micropores of less than 0.4 nm opening. This conclusion is

corroborated by the fact that if the selectivity was due to

micropores, the high-affinity sites should account for more

than 15% of the total sites and in accordance to the ratio of

the micropore volume to the total volume (Hutson et al.,

1998). Moreover, an increase in the micropore volume of less

than 0.4 nm opening has been observed with increasing ionic

radius of the post exchange cation (e.g. cesium) (Hutson et al.,

1998). In this case, the cesium loading should lead to higher

amount of pores with opening less than 0.4 nm and subse-

quently higher selectivity. This result was not observed in the

present study. Finally, increasing the pillaring density and

therefore the fraction of micropores, did not result in higher

selectivity coefficients.

3.3. Copper sorption experiments

3.3.1. Copper sorption on different PILMsConsidering the 0.96 nm layer thickness of montmorillonite

and the 0.54 nm diameter of the hydrated copper cation

CuðH2OÞ2þ6 (Kukkadapu and Kevan, 1988), a minimum d001-

spacing of 1.50 nm is necessary for the existence of the

CuðH2OÞ2þ6 species in PILMs interlayer space. Thus, the

hydrated copper ion can enter the interlammelar space of

all the prepared aluminum-pillared montmorillonites. In this

frame, sorption of copper (0.5 mM) on the prepared PILM

materials was studied in aqueous solutions without or with

the presence of competing cations of Hþ, Kþ or Ca2þ in

concentrations similar to that found in drinking water

(1.5 mM).

As seen in Table 3, the Al2MFD15 sample exhibited the

higher sorption ability of all the prepared materials and its

PILMs (total concentration ¼ 0:5 mM)

Al1MFD3 Al2MFD5 Al2MFD15-Una Al2MFD15

42:8� 1:8 67� 2:5 20:7� 1:6 95:1� 2:9

1.47 1.49 1.61

t restored after pillaring.

pacity (CEC) of the sample.

ARTICLE IN PRESS

Fig. 9 – Copper (0.5 mM) sorption kinetics by NaM and

Al2MFD15.

Fig. 10 – Effect of pH on copper sorption (0.5 mM) on NaM

and Al2MFD15.

WAT E R R E S E A R C H 4 1 ( 2 0 0 7 ) 1 8 9 7 – 1 9 0 61904

sorption capacity was higher than the cation-exchange

capacity of the starting clay material. Assuming the equality

of the maximum cesium sorption capacity and the cation-

exchange capacity of the PILMs materials (Karamanis et al.,

1997), the second row in Table 3 indicates that the amount of

Cu2þ sorbed on PILMs increased with higher pillar density.

Therefore, copper sorption on PILMs involves both a cation-

exchange process in the surface and interlammelar clay sites

and complexation reactions with the pillar oxides. This

conclusion is further corroborated from the observation that

the PILM sample with unrestored cation-exchange capacity,

sorbed 0.21 meq of copper per gram of material. This amount

is mainly bound to pillar oxides since PILMs’ cation-exchange

capacity is drastically reduced after the pillaring process

(Karamanis et al., 1997).

Furthermore, a reduction of 21.3% and 15.3% of sorbed

copper by Al2MFD15 was observed with the addition of

potassium or calcium ions as competitive cations. Both

reductions can be attributed to the reduction of the sorption

fraction of the interlayer sites group due to PILMs preference

for the sorption of monovalent cations as potassium over

divalent as calcium. This result was also observed in cesium

sorption. Finally, the Al2MFD15 sample with a solution to

solid ratio (V=M) of 1 L g�1, removed 99.7% of an initial copper

concentration of 32 mg L�1 in natural mineral water, reducing

in this way the copper level in a value much lower than the

maximum acceptable level of 2 mg L�1.

3.3.2. Copper sorption kineticsCopper sorption on the PILM material with the highest Al/clay

ratio (Al2MFD15) was fast and was accomplished within

20 min (Fig. 9). After testing different models, the copper

sorption by PILMs was better described by the Lagergren

pseudo-first-order equation. The rate constant k1 (min�1) and

the equilibrium capacity, qe ðmmol g�1Þ determined by fitting

Eq. (2) to the experimental results are included in Fig. 9. The

rate constant of copper sorption on the pillared sample was

higher than the starting montmorillonite. This result can be

attributed to the easiness of pores accessibility due to the

three-dimensional structure of PILMs than the blocking of

pores after the initial sorption of copper within the mon-

tmorillonite interlayer space and the subsequent collapse of

the aluminosilicate clay sheets. Concluding, it is clear from

the kinetic measurements that the velocity of transport of

copper from the liquid phase to solid phase is rapid enough

for PILMs application purposes in the treatment of polluted

aqueous solutions.

3.3.3. Effect of pHThe effect of the pH on the copper sorption by PILMs was

studied in the pH region between 3.0 and 6.0. The pH was

limited to values less than 6 because of the formation of

copper hydroxyl species at higher pH as determined by the

visual MINTEQ code (Gustafsson, 2003). As seen in Fig. 10, the

sorption of copper ions increased around 30% with an

increase in pH of the solution from 3.0 to 4.0 and then

remains constant. As it has been observed in similar studies

with PILMs, the increase of pH decreases the competition

between the protons and the metal ions for surface sites and

results in increased metal uptake by the PILM (Manohar et al.,

1

2006). Finally, copper sorption on the Al2MFD15 sample was

much higher than the removal by montmorillonite in all the

studied pH range.

3.3.4. Copper sorption isothermsThe equilibrium data for copper sorption covered the

concentration range from 10 to 100 mg L�1 and were subjected

to the sorption isotherms of Langmuir and Freundlich

(Fig. 11). The Langmuir model parameters and the statistical

fits of the sorption data to Eqs. (3) and (4) are given in Table 4.

The one-site Langmuir isotherm appeared to be inadequate to

describe the sorption behavior of PILMs because it under-

estimated sorption at low initial concentrations. The high

SD(%) value of the one-site Langmuir model was dramatically

reduced with the application of the Freundlich model. The

Freundlich model is characterized by 1=n, the heterogeneous

factor, hence it is applicable to sorption on heterogeneous

surfaces, i.e., surface with non-energetically equivalent sites.

The fact that the Freundlich isotherm fits the experimental

data very well can be explained from the heterogeneous

distribution of active sites on the PILM materials. Indeed, the

two-site Langmuir model described copper sorption onto

ARTICLE IN PRESS

Fig. 11 – Comparison of the Langmuir and Freundlich

isotherms for the sorption of copper onto NaM and

Al2MFD15 (qe is the adsorption capacity at equilibrium

(mg g�1) and Ce is the equilibrium concentration of metal in

the solution (mg L�1)).

Table 4 – Freundlich and Langmuir isotherm constants

for the sorption of Cu2þ onto Al2MFD15

Values and parameters Magnitude

Freundlich

KF (L g�1Þ 19:53� 1:02

1=n 0:112� 0:006

R2 0.995

SD(%) 2.78

One-site Langmuir

Q ðmg g�1Þ 32:92� 1:41

K ðL mg�1Þ 0:394� 0:275

R2 0.998

SD(%) 43.84

Two-site Langmuir

Q1 ðmg g�1Þ 23:89� 4:36

K1 ðL mg�1Þ 0:011� 0:004

Q2 ðmg g�1Þ 22:12� 0:50

K2 ðL mg�1Þ 15:97� 1:69

w2 0.07

SD(%) 0.45

WAT E R R E S E A R C H 41 (2007) 1897– 1906 1905

1

PILMs even better. The determined constants showed that

high and low affinity sites exist with almost the same

capacity and with a value half to the sorption maximum

(0:72 mmol g�1Þ. These two different affinity sites can be

attributed to the pillar oxides and to typical exchange sites

compensating the negative charge of the clay sheets. The last

are restored after the pillaring process. In desorption studies,

the Cu2þ ions were detected in negligible quantities for all

samples when the desorption was carried out using deionized

water. When the supernatant was replaced by CaCl2 or KCl

solutions, very small amounts of Cu2þ were detected in the

solutions. In contrast, by lowering the pH from about 5–6 after

the end of sorption to around 1.8, almost all the sorbed copper

was released in the solution.

4. Conclusion

The results of this study indicate that Al-pillared montmor-

illonites are potential sorbents for the removal of cesium or

copper from aqueous solutions. The most effective pH range

was found to be 4.0–6.0 for the removal of copper and 3.0–8.0

for cesium. The sorption of either cesium or copper follows

the pseudo-first-order sorption reaction while the sorption

isotherms follow the two-site Langmuir model. Sorption

experiments with blocking agents revealed that complemen-

tary to the interlayer clay sites, a small fraction of PILMs sites

exist (1–2%) that is very selective for cesium sorption over

competitive monovalent and divalent inorganic and organic

cations. These sites are related to the surface or edges of the

PILM materials and their fraction is independent on PILMs

d001 spacing or the Al/clay ratio used in their preparation. In

contrast, the Al/clay ratio appears to have a significant effect

on the sorption of copper on PILMs. With increasing Al/clay

ratio, the amount of sorbed copper increases. This indicates

that the sorption of copper involves specific group of high-

affinity sites on the pillar surfaces in addition to the low-

affinity restored clay interlayer sites of the PILM materials.

Therefore, copper sorption should be driven by both a cation-

exchange mechanism and by complexation reactions with

the pillar oxides.

Acknowledgments

This work was partially supported by the Empirikion Founda-

tion. The authors thank Dr. N. Kourkoumelis of the XRD Unit

of the University of Ioannina.

R E F E R E N C E S

Al-Qunaibit, M.H., Mekhemer, W.K., Zaghloul, A.A., 2005. Theadsorption of Cu(II) ions on bentonite—a kinetic study.J. Colloid Interface Sci. 283, 316–321.

Apak, R., Atun, G., Guc- lu, K., Tutem, E., 1996. Sorptive removal ofcesium-137 and strontium 90 from water by unconventionalsorbents. II. Usage of coal fly ash. J. Nucl. Sci. Technol. 33,396–402.

Avena, M.J., Cabrol, R., De Paulil, C.P., 1990. Study of somephysicochemical properties of pillared montmorillonites:acid–base potentiometric titrations and electrophoretic mea-surements. Clays Clay Miner. 38, 356–362.

Babel, S., Kurniawan, T.A., 2003. Low-cost adsorbents for heavymetals uptake from contaminated water: a review. J. Hazard.Mater. 45, 219–243.

Bailey, S.E., Olin, T.J., Bricka, R.M., Adrian, D.D., 1999. A review ofpotentially low-cost sorbents for heavy metals. Water Res. 33,2469–2479.

Cooper, C., Jiang, J.Q., Ouki, S., 2002. Preliminary evaluation ofpolymeric Fe- and Al-modified clays as adsorbents for heavymetal removal in water treatment. J. Chem. Technol. Bio-technol. 77, 546–551.

Cremers, A., Elsen, A., De Preter, P., Maes, A., 1988. Quantitativeanalysis of radiocesium retention in soil. Nature 335, 247–249.

Delgado, A., Gonzalez-Caballero, F., Bruque, J.M., 1986. On the zetapotential and surface charge density of montmorillonite in

ARTICLE IN PRESS

WAT E R R E S E A R C H 4 1 ( 2 0 0 7 ) 1 8 9 7 – 1 9 0 61906

aqueous electrolyte solutions. J. Colloid. Interface Sci. 113,203–211.

Gil, A., Montes, M., 1995. Analysis of the microporosity in pillaredclays. Langmuir 10, 291–297.

Grim, R.E., 1968. Clay Mineralogy. McGraw-Hill, New York, p. 214(Chapter 7).

Gurboga, G., Tel, H., 2005. Preparation of TiO2–SiO2 mixed gelspheres for strontium adsorption. J. Hazard. Mater. 120,135–142.

Gustafsson, J.P., 2003. Visual MINTEQ (VMINTEQ) Version 2.14;KTH, Department of Land and Water Resources Engineering,Stockholm, Sweden.

Hutson, N.D., Gualdoni, D.J., Yang, R.T., 1998. Synthesis andcharacterization of the microporosity of ion-exchangedAl2O3-pillared clays. Chem. Mater. 10, 3707–3715.

Hutson, N.D., Hoekstra, M.J., Yang, R.T., 1999. Control of micro-porosity of Al2O3-pillared clays: effect of pH, calcinationtemperature and clay cation exchange capacity. Microp.Mesop. Mater. 28, 447–460.

Ioannidis, S., Anderko, A., Sanders, S.J., 2000. Internally consistentrepresentation of binary ion exchange equilibria. Chem. Eng.Sci. 55, 2687–2698.

Jiang, J.Q., Zeng, Z., 2003. Comparison of modified montmorillo-nite adsorbents. Part II: the effects of the type of raw clays andmodification conditions on the surface properties and ad-sorption performance of modified clays. Chemosphere 53,53–62.

Karamanis, D., Aslanoglou, X., Assimakopoulos, P.A., Gangas,N.H., 2001. PIGE and XRF analysis of a nano-composite pillaredlayered clay material for nuclear waste applications. Nucl.Instrum. Methods B 181, 616–621.

Karamanis, D.T., Aslanoglou, X., Assimakopoulos, P.A., Gangas,N.H., Pakou, A., Papayanakos, N., 1997. An aluminum pillaredmontmorillonite with fast uptake of cesium and strontiumfrom aqueous solutions. Clays Clay Miner. 45, 709–717.

Kukkadapu, R.K., Kevan, L., 1988. Synthesis and electron spinresonance studies of copper-doped alumina-pillared mon-tmorillonite clay. J. Phys. Chem. 92, 6073–6078.

Lahiri, S., Roy, K., Bhattacharya, S., Maji, S., Basu, S., 2005.Separation of 134Cs and 152Eu using inorganic ion exchangers,zirconium vanadate and ceric vanadate. Appl. Radiat. Isot. 63,293.

1

Lin, Y., Fryxell, G.E., Wu, H., Engelhard, M., 2001. Selective sorptionof cesium using self-assembled monolayers on mesoporoussupports (SAMMS). Environ. Sci. Technol. 35, 3962–3966.

Lothenbach, B., Furrer, G., Schulin, R., 1997. Immobilization ofheavy metals by polynuclear aluminium and montmorillonitecompounds. Environ. Sci. Technol. 31, 1452–1462.

Manohar, D.M., Noeline, B.F., Anirudhan, T.S., 2005. Removal ofvanadium(IV) from aqueous solutions by adsorption processwith aluminum-pillared bentonite. Industrial & EngineeringChemistry Research 44, 6676–6684.

Manohar, D.M., Noeline, B.F., Anirudhan, T.S., 2006. Adsorptionperformance of Al-pillared bentonite clay for the removal ofcobalt(II) from aqueous phase. Appl. Clay Sci. 31, 194–206.

Mishael, Y.G., Rytwo, G., Nir, S., Crespin, M., Bergaya, F.A., VanDamme, H., 1999. Interactions of monovalent organic cationswith pillared clays. J. Colloid Interface Sci. 209, 123–128.

Pendelyuk, O.I., Lisnycha, T.V., Strelko, V.V., Kirillov, S.A., 2005.Amorphous MnO2–TiO2 composites as sorbents for Sr2þ andUO2þ

2 . Adsorption 11, 799–804.Reddad, Z., Gerente, C., Andres, Y., Le Cloirec, P., 2002. Adsorption

of several metal ions onto a low cost biosorbent: kinetic andequilibrium studies. Environ. Sci. Technol. 36, 2067–2073.

Solbra, S., Allison, N., White, S., Mikhalovsky, S., Bortun, A.I.,Bortun, L.N., Clearfield, A., 2001. Cesium and strontium ionexchange on the framework titanium silicate M2Ti2O3SiO4 �

nH2O (M ¼ H, Na). Environ. Sci. Technol. 35, 626.Staunton, S., Roubaud, M., 1997. Adsorption of 137Cs on mon-

tmorillonite and illite: effect of charge compensating cation,ionic strength, concentration of Cs, K, and fulvic acid. ClaysClay Miner. 45, 251–260.

Stout, S.A., Cho, Y., Komarneni, S., 2006. Uptake of cesium andstrontium cations by potassium-depleted phlogopite. Appl.Clay Sci. 31, 306–313.

Varma, R.S., 2002. Clay and clay-supported reagents in organicsynthesis. Tetrahedron 58, 1235–1255.

Wang, Y.H., Lin, S.H., Juang, R.S., 2003. Removal of heavy metalions from aqueous solutions using various low-cost adsor-bents. J. Hazard. Mater. B 102, 291–302.

Zachara, J.M., Smith, S.C., Liu, C., McKinley, J.P., Serne, R.J.,Gassman, P.L., 2002. Sorption of Csþ to micaceous subsurfacesediments from the Hanford site, USA. Geochim. Cosmochim.Acta 66, 193–211.